In general, in an electrophotographic image forming apparatus, an exposure device exposes a surface of a photoreceptor to form an electrostatic latent image, a developing device develops the formed electrostatic latent image to form a toner image, and the formed toner image is transferred to a recording medium, so that an image is formed.
In the photoreceptor on which the electrostatic latent image is formed, irregularity of charging characteristics or sensitivity characteristics exists in an axial direction of a drum surface due to factors on manufacturing. Furthermore, when the toner image is formed by developing the electrostatic latent image, there is a problem that density irregularity occurs in a main scanning direction (a drum shaft direction) due to an influence of variance of a gap with a developer carrier that supplies toner to the photoreceptor, variance of a light amount of an exposure means in the main scanning direction, and the like.
In order to solve such a problem, in an image forming apparatus disclosed in Patent Literature 1, a belt-like patch image extending in the whole main scanning direction of an image forming area is printed, and an exposure amount is corrected at each position in the main scanning direction such that a density at each position of the printed patch image in the main scanning direction reaches a target density. When the exposure amount is corrected, it is necessary to calculate a density change rate to the exposure amount. In this regard, in the image forming apparatus disclosed in Patent Literature 1, two patch images having different exposure amounts are printed and densities of both images are compared with each other at each position in the main scanning direction, so that the density change rate to the exposure amount is calculated (see paragraph 0060 of Patent Literature 1).